The storage of electrical energy has become increasingly important in recent decades. Electrical energy may be stored with the aid of batteries. Batteries convert chemical reaction energy into electrical energy. A distinction is made between primary batteries and secondary batteries. Primary batteries are non-rechargeable, while secondary batteries, also referred to as accumulators, are rechargeable. A battery includes one or multiple battery cells.
In particular, so-called lithium-ion battery cells are used in an accumulator. They are characterized, among other features, by high energy densities, thermal stability, and extremely low self-discharge.
Lithium-ion battery cells include a positive electrode and a negative electrode. The positive and negative electrodes each include a current collector, to which a positive or negative active material is applied.
The positive and negative active material is capable of reversible intercalation and deintercalation of lithium ions.
The active material for the negative electrode is, for example, amorphous silicon which may form alloy compounds with lithium atoms. However, carbon compounds such as graphite are also widely used as active material for negative electrodes. Lithium atoms are intercalated into the active material of the negative electrode.
A lithium-containing metal oxide or a lithium-containing metal phosphate is generally used as active material for the positive electrode. In particular in applications in which a high energy density is necessary, so-called high-energy materials such as high-energy (HE) nickel-cobalt-manganese (NCM) electrodes (for example, LiMO2:Li2MnO3, where M=Ni, Co, Mn) are used. A generic battery that uses such an HE-NCM electrode is described in German Patent Application No. DE 10 2012 208 321 A1, for example.
During operation of the battery cell, i.e., during a discharging operation, electrons flow in an external circuit from the negative electrode to the positive electrode. During a discharging operation, lithium ions migrate from the negative electrode to the positive electrode within the battery cell. In the process, the lithium ions are reversibly deintercalated from the active material of the negative electrode, also referred to as delithiation. During a charging operation of the battery cell, the lithium ions migrate from the positive electrode to the negative electrode. In the process, the lithium ions are reversibly reintercalated into the active material of the negative electrode, also referred to as lithiation.
The electrodes of the battery cell have a foil-like design and are wound to form an electrode winding, with a separator situated in between which separates the negative electrode from the positive electrode. Such an electrode winding is also referred to as a “jelly roll.” The electrodes may also be layered one above the other to form an electrode stack.
The two electrodes of the electrode winding or of the electrode stack are electrically connected with the aid of collectors to poles of the battery cell, also referred to as terminals. A battery cell generally includes one or multiple electrode windings or electrode stacks. The electrodes and the separator are surrounded by an electrolyte composition which is generally liquid. The electrolyte composition is conductive for the lithium ions, and allows transport of the lithium ions between the electrodes.
U.S. Patent Appl. Pub. No. 2014/0242468 A1 describes a positive active material that contains a superlithiated lithium-transition metal oxide, a portion of the lithium ions having been replaced by transition metal ions.
U.S. Patent Appl. Pub. No. 2014/0147727 A1 describes an active material that includes a lithium-manganese oxide of formula x MnO2.(1−x) Li2MnO3 (where 0<x<1) and a lithium-containing transition metal oxide such as Li2NixCu1-xO2 (where 0≤x≤1).
Battery cells that include conventional HE-NCM electrodes are characterized in that they deliver high cell voltages at the beginning of the service life of the cell, but over the service life are subject to significant losses (so-called “voltage fade”). The same applies for the capacity of the cell (so-called “capacity fade”). The object of the present invention, therefore, is to provide an active material for a positive electrode, which has a high cell voltage and capacity even after a long service time of the cell.